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author | Ken Raeburn <raeburn@mit.edu> | 2001-04-10 00:47:01 +0000 |
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committer | Ken Raeburn <raeburn@mit.edu> | 2001-04-10 00:47:01 +0000 |
commit | f49fb3ce569bfb75a3175a73e8078f2404527491 (patch) | |
tree | 95f33f96202e9542bc4078ce9e2589ca0e6c2fa8 /src/lib/des425 | |
parent | e8855363194dc68093492a4ec6fb93805d4174ce (diff) | |
download | krb5-f49fb3ce569bfb75a3175a73e8078f2404527491.zip krb5-f49fb3ce569bfb75a3175a73e8078f2404527491.tar.gz krb5-f49fb3ce569bfb75a3175a73e8078f2404527491.tar.bz2 |
* quad_cksum.c (des_quad_cksum): Add comments. Force 32-bit arithmetic just to be careful
git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@13148 dc483132-0cff-0310-8789-dd5450dbe970
Diffstat (limited to 'src/lib/des425')
-rw-r--r-- | src/lib/des425/ChangeLog | 5 | ||||
-rw-r--r-- | src/lib/des425/quad_cksum.c | 41 |
2 files changed, 41 insertions, 5 deletions
diff --git a/src/lib/des425/ChangeLog b/src/lib/des425/ChangeLog index 2cc9e3f..3274772 100644 --- a/src/lib/des425/ChangeLog +++ b/src/lib/des425/ChangeLog @@ -1,3 +1,8 @@ +2001-04-09 Ken Raeburn <raeburn@mit.edu> + + * quad_cksum.c (des_quad_cksum): Add comments. Force 32-bit + arithmetic just to be careful. + 2001-04-07 Ken Raeburn <raeburn@mit.edu> * t_quad.c: New file. diff --git a/src/lib/des425/quad_cksum.c b/src/lib/des425/quad_cksum.c index 4ae542f..e9340bd 100644 --- a/src/lib/des425/quad_cksum.c +++ b/src/lib/des425/quad_cksum.c @@ -23,7 +23,10 @@ * M.I.T. makes no representations about the suitability of * this software for any purpose. It is provided "as is" without express * or implied warranty. - * + * + * + * This routine does not implement: + * * * Quadratic Congruential Manipulation Dectection Code * @@ -35,8 +38,7 @@ * This routine, part of the Athena DES library built for the Kerberos * authentication system, calculates a manipulation detection code for * a message. It is a much faster alternative to the DES-checksum - * method. No guarantees are offered for its security. Refer to the - * paper noted above for more information + * method. No guarantees are offered for its security. * * Implementation for 4.2bsd * by S.P. Miller Project Athena/MIT @@ -82,6 +84,22 @@ * cant get at the carry or high order results from multiply, * but nontheless is 64 bit arithmetic. */ +/* + * This code purports to implement the above algorithm, but fails. + * + * First of all, there was an implicit mod 2**32 being done on the + * machines where this was developed because of their word sizes, and + * for compabitility this has to be done on machines with 64-bit + * words, so we make it explicit. + * + * Second, in the squaring operation, I really doubt the carry-over + * from the low 31-bit half of the accumulator is being done right, + * and using a modulus of 0x7fffffff on the low half of the + * accumulator seems completely wrong. And I challenge anyone to + * explain where the number 83653421 comes from. + * + * --Ken Raeburn 2001-04-06 + */ /* System include files */ @@ -144,6 +162,10 @@ des_quad_cksum(in,out,length,out_count,c_seed) len = length; p = in; while (len) { + /* + * X = Z + Input ... sort of. Carry out from low half + * isn't done, so we're using all 32 bits of x now. + */ if (len > 1) { x = (z + vaxtohs(p)); p += 2; @@ -154,8 +176,17 @@ des_quad_cksum(in,out,length,out_count,c_seed) len = 0; } x2 = z2; - z = ((x * x) + (x2 * x2)) % 0x7fffffff; - z2 = (x * (x2+83653421)) % 0x7fffffff; /* modulo */ + /* + * I think this is supposed to be a squaring operation. + * What it really is, I haven't figured out yet. + * + * Explicit mod 2**32 is for backwards compatibility. Why + * mod 0x7fffffff and not 0x80000000 on the low half of + * the (supposed) accumulator? And where does the number + * 83653421 come from?? + */ + z = (((x * x) + (x2 * x2)) & 0xffffffff) % 0x7fffffff; + z2 = ((x * (x2+83653421)) & 0xffffffff) % 0x7fffffff; /* modulo */ #ifdef DEBUG if (des_debug & 8) printf("%d %d\n",z,z2); |